AC and heating stopped working

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analogo

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Hello,

I noticed both my AC and heating recently stopped working (probably around the same time). I have a mostly original 72 AC/heater system with a new heater core installed a few years ago and it worked perfectly up to a few weeks ago.

The blower still works and changing the control lever correctly routes air to the defrost, vents and floor which suggest there is vacuum and those motors are actuated correctly.
What I can't figure out is why I do not get any cold or hot air regardless of where the temperature lever is.
I understand AC and heating are separate systems so I assume the likeliest option is that a shared element is causing the problem. I wonder if the water valve vacuum switch or motor might be the cause, but any ideas are welcome!
 

Hemikiller

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Are you getting air flow out the vents? If not, then check the AC fuse in the #5 slot in the fuse box, it's a 30amp glass unit. If that's good, then you need to troubleshoot the blower motor circuit from the fan switch through the resistor, to the motor.

The temperature is controlled by a cable from the back of the HVAC control to the blend door. When you move the temperature lever, it should have resistance and you will be able to feel and/or hear the door moving.


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analogo

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Thanks Hemikiller,
Air is coming out of the vents and the fuse seems to be ok. When I move the temperature lever, I do feel resistance and there is a sound of something moving as well.
 

Hemikiller

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OK, so the temperature doesn't change when you move the lever?

For warm air, coolant has to flow through the heater core. There is a flow control valve on the heater hose that closes when the mode selector is placed into the AC position. Those can get corroded with age and stick closed, that would prevent you from having hot air. Your heater core could be disconnected, or plugged as well.

For cold air, the AC unit has to engage the compressor. The mode selector triggers a microswitch that turns the compressor on, after it passes through the deicing switch. You can check for voltage at the compressor clutch. If it's present, then something is up with the clutch. If not, then you need to diagnose from the clutch back to the mode control.

Link to wiring diagrams below.

 

analogo

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As Hemikiller suggested, it seems that the heater water valve was stuck in the closed position and disconnected completely from the vacuum line (the vacuum line was also plugged with a screw, I wonder if someone in the shop sabotaged it)

I manually moved the valve and now I'm able to get hot air when "warm" setting and warm air when "cold". I assume the warm air I get in the "cold" setting is being caused by the same valve not closing when it should.

Tested the voltage at the compressor and it does not seem to be receiving anything. I'll check the switch in the control panel later.
 
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As Hemikiller suggested, it seems that the heater water valve was stuck in the closed position and disconnected completely from the vacuum line (the vacuum line was also plugged with a screw, I wonder if someone in the shop sabotaged it)

I manually moved the valve and now I'm able to get hot air when "warm" setting and warm air when "cold". I assume the warm air I get in the "cold" setting is being caused by the same valve not closing when it should.

Tested the voltage at the compressor and it does not seem to be receiving anything. I'll check the switch in the control panel later.
The valve can be “finicky” if older and even if new, corrosion from heater box may flake off and create a clogged “artery” in the valve when the vacuum actuates it. IMO, if it was working one day and then stopped w/o a clutch noise sound pitched higher (as struggling and not over amped - (blown fuse) then a complete douching of the entire system may be a solution. You said the fuse “seams” fine. Its either still linked or not, so MO is change it (may as well change them all while your upside down under there) with clear glass fuses and look for any corrosion, loose grounds, mice nests…. etc.,
I would hope sabotage is not a logical cause. If you believe that then change shops or verify the actual root cause and keep us posted. 👌🏻
 
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